This invention relates generally to radio frequency signal receivers and more particularly to heterodyne receivers used to determine the frequency of a received signal.
It is well known in the art that the modulation components of radio frequency (RF) signals may be amplified and then detected after "down conversion" in a conventional heterodyne receiver. In the ordinary heterodyne receiver designed for use when the carrier frequency of the received signal is, a priori, not known, at least the first intermediate local oscillator is arranged to be tunable. Thus, as is well known, a predetermined difference signal (usually referred to as the I.F. signal) may be produced by mixing the local oscillator signal with a received RF signal. The modulation components of the RF signal then are translated to identical modulation components on an I.F. carrier. Such translated modulation components, after being passed through an I.F. amplifier (which may also be considered to be a bandpass filter) may then be detected.
In many applications, a conventionally tuned heterodyne receiver is not satisfactory. For example, if the frequency of a radio frequency pulse signal is not known, the time taken to tune the local oscillator may be too great. Also, another shortcoming of a conventional heterodyne receiver derives from the contrary requirements for ease of tuning and resolution between different radio frequency signals. The requirement for the former is that the bandpass of the I.F. amplifier be as wide as possible while the requirement for the latter is that the bandpass of the I.F. amplifier be, ideally, no wider than the band of the modulation components. There are many known ways in which the conflicting requirements may be met. For example, a second conversion stage may be added to the receiver such that a second local oscillator is varied through a band of frequencies displaced from a first intermediate frequency band to produce a second down converted signal, or an intermediate frequency signal may be impressed on a comb filter and the energized one, or ones, of the channels therein may be detected. The latter approach, however, is limited for a fixed number of filters in the comb filter. For example, to cover a frequency band of 2-4 GHz with a frequency resolution of 4 MHz, 500 filters in the comb filter would be required.